Membrane switches comprising a plurality of switches have been used in the past to input information. In general, membrane switches comprise a circuit or matrix layout embedded in a plastic film. Switches, generally comprising domes, are located at specific locations in the plastic film. The matrix layout comprises a plurality of lines arranged in columns and rows upon the membrane. The switches are located at the intersection of a column and row line in the matrix layout such that when the switches are activated, the two lines in the matrix layout become electrically connected, decreasing the resistance between the two lines and thereby indicating that the corresponding switch has been activated. Domed switches can be activated by pressing down on the switch to electrically connect a row line with a column line.
In general, the columns and rows of the matrix layout must exit the membrane from the same location so that the output signals generated by activation of the switches can be easily taken from the membrane switch to a processing device. The processing device is usually located remotely from the membrane switch.
The column and row lines of the matrix layout end in output lines extending from the membrane switch. Pins can be attached to the output lines so that a cable having a connector at one end can be attached to the pins. The other end of the cable can then be attached to the processing device.
The processing device is generally the motherboard or logic board of an electrical element within which the membrane switch and motherboard are contained. The electrical element can be, for example, an electrical appliance, such as a microwave oven, with a membrane switch input/output system to allow the user to input information which is then sent to the motherboard of the microwave oven to operate the oven. The motherboard or logic board will generally contain a microprocessor to receive the signals from the membrane switch and send out control signals to operate the electrical appliance.
One disadvantage of the prior art systems is that the matrix layout generally has several columns and rows and therefore has a large number of output lines exiting from the membrane switch. Therefore, any cable which connects the membrane switch to the motherboard must have a separate line or wire for each output line of the membrane switch to transfer the output signal from the membrane switch to the motherboard. Clearly, this increases the cost of the system by requiring a more expensive cable. Also, this requires more space on the motherboard to connect the cable to the motherboard. Furthermore, several pins on the input/output chips of the motherboard must be dedicated to receiving the output signals from the membrane switch. Both of these features increase the cost and complexity of the motherboard.
A further disadvantage of the prior art systems is that the cable, connectors and output lines generally increase the resistance of the membrane switch. This increased resistance corrupts the signal by increasing noise and decreasing the signal during transmission from the membrane switch to the motherboard.
In addition, the switches on membrane switches suffer from "key rollover" because the switch does not cleanly connect the row and column lines, thereby often creating multiple erroneous signals upon each activation of a switch. However, the membrane switch has no means for cleaning the signal or debouncing the signal to end key rollover. While these problems could be corrected at the motherboard, this again increases the cost of the motherboard. Furthermore, if the manufacturer of the membrane switch is not the same as the manufacturer of the motherboard, the manufacturer of the motherboard may not know what type of filtering or signal processing are required for the membrane switch.
Some prior art membrane switches comprise LEDs which can be lit in response to activation of switches. However, to operate such LEDs, a transistor must be incorporated in the membrane switch to drive each LED, which increases the cost of the membrane switch. The prior art membrane switches do not have a means for activating and powering an LED located on the membrane switch from a location off of the membrane switch, but not on the motherboard.
In addition, many processing devices, such as motherboards, are designed to accept a predetermined code, meaning that a predetermined combination of signals on the lines being inputted to the motherboard identify activation of a specific switch on the membrane switch. Generally, the manufacturer of the motherboard would not also be the manufacturer of the membrane switch. Therefore, the manufacturer of the membrane switch must arrange the matrix layout to conform with the predetermined code required by the processing device. This often increases the complexity of the matrix layout in the membrane switch by requiring lines in the matrix to cross over one another. Each time a line in a membrane switch crosses over another line, a bridge must be inserted in the membrane layout to avoid short-circuiting the two lines, which increases the cost of manufacture of the membrane switch. This problem is compounded when the matrix layout must avoid a display area in the membrane switch through which the lines forming a matrix layout cannot pass. In either case, the membrane switches must be customized so that the matrix layout provides output signals in the code required by the processing device. This customization increases costs by requiring several different types of membrane switches to be designed and manufactured. Furthermore, an existing membrane switch cannot be altered or re-wired. Therefore, an existing membrane switch cannot be changed or re-wired to meet new requirements, but rather must be replaced if its code is incorrect or the predetermined code which the processing device will accept has changed.